So why do we fast before Dormition? In a close-knit family, word that its matriarch is on her deathbed brings normal life to a halt. Otherwise important things (parties, TV, luxuries, personal desires) become unimportant; life comes to revolve around the dying matriarch. It is the same with the Orthodox family; word that our matriarch is on her deathbed, could not (or at least should not) have any different effect than the one just mentioned. The Church, through the Paraklesis Service, gives us the opportunity to come to that deathbed and eulogize and entreat the woman who bore God, the vessel of our salvation and our chief advocate at His divine throne. And as, in the earthly family, daily routines and the indulgence in personal wants should come to a halt. Fasting, in its full sense (abstaining from food and desires) accomplishes this. Less time in leisure or other pursuits leaves more time for prayer and reflection on she who gave us Christ, and became the first and greatest Christian. In reflecting on her and her incomparable life, we see a model Christian life, embodying Christ’s retort to the woman who stated that Mary was blessed because she bore Him: blessed rather are those who hear His word and keep it. Mary did this better than anyone. As Fr. Thomas Hopko has stated, she heard the word of God and kept it so well, that she of all women in history was chosen not only to hear His Word but give birth to it (Him). So while we fast in contemplation of her life, we are simultaneously preparing ourselves to live a life in imitation of her. That is the purpose of the Dormition Fast.
SOURCE:Antiochian Orthodox Christian Archdiocese By Fr. Gabriel Barrow